1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring a level of a liquid in a vessel. The present invention also relates to an apparatus for manufacturing a semiconductor device. More particularly, the present invention relate to a method of and an apparatus for measuring a level of a liquid that is converted to a gas used in the manufacturing of a semiconductor device.
2. Description of the Related Art
In general, a semiconductor device is manufactured by performing and repeating several individual processes on a substrate to form a series of patterns on the substrate. These processes include a deposition process, a photolithography process, an etching process, a chemical mechanical polishing (CMP) process, a washing process, and a drying process. The deposition process is performed to form a layer on the substrate. The photolithography and etching processes are performed to pattern the layer, e.g., to form a line type of pattern comprising a series of lines and/or a contact type of pattern comprising a series of contact holes. The deposition process increases in importance as the smaller the critical dimension of, for example, the lines of the line type of pattern to be formed on the substrate becomes and/or the higher the aspect ratio of, for example, the contact holes of the contact type of pattern becomes.
Methods of forming the layer on the substrate of the semiconductor device generally include chemical vapor deposition (CVD), low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), and metal organic chemical vapor deposition (MOCVD). Additionally, a cyclic chemical deposition (CCVD) process, a digital chemical vapor deposition (DCVD) process, and an advanced chemical vapor deposition (ACVD) process can be employed for forming a layer on the substrate.
The above-mentioned methods of forming a layer on a substrate all employ a gas to provide an element of the material of the layer. This gas and a reactant such as a metal organic precursor or a metal halide are supplied to the substrate as source gases. In order to minimize impurities in the layer, an organic ligand or a halide respectively coupled with a metal element in the reactant on the substrate is removed by decomposition in the chemical vapor deposition process, and is removed by chemical exchange in the atomic layer deposition process. That is, in chemical vapor deposition, the source gases are mixed in a process chamber and are supplied onto a surface of a substrate disposed in the chamber. On the other hand, in atomic layer deposition, the source gases are not mixed in the process chamber. Rather, the source gases are supplied in respective pulses onto the substrate. For example, first, only a first source gas is supplied into the process chamber to thereby be chemically absorbed on the substrate. Then a second source gas is supplied into the process chamber to thereby be chemically coupled on the substrate. Generally, a liquid is evaporated to produce the source gas, and the source gas is supplied into the process chamber using a carrier gas.
U.S. Pat. No. 6,155,540 (issued to Hwang et al.) discloses an apparatus that evaporates a liquid and supplies the vapors as a source gas in a chemical vapor deposition process. In this apparatus, the liquid is fed to a vaporizer at a rate controlled by the apparatus, is atomized by an ultrasonic atomizing device, and is evaporated by a heated carrier gas in the vaporizer.
In another known type of method of producing source gas from a liquid, the liquid is stored in a canister and the heated carrier gas is bubbled through the reservoir of the liquid. In this method, the canister is replaced after a predetermined amount of the liquid is used up. More specifically, a level of the liquid in the canister is measured by a liquid level detector. The canister is replaced once the liquid drops below a predetermined level in the canister.
One way for measuring the level of a liquid stored in a vessel is to detect the change in pressure in the vessel as the liquid evaporates. Korean Laid-Open Patent Publication No. 1999-32720 discloses an apparatus of this type for measuring the amount of fuel stored in a vessel. An advantage of this apparatus is that the amount of liquid can be measured precisely irrespective of a shape of the vessel.
FIG. 1 is a schematic cross-sectional view of a conventional apparatus for measuring a level of a liquid stored in a vessel. Referring to FIG. 1, the apparatus comprises a first guide pole 20 and a second guide pole 22 each extending from an upper surface of a vessel 10 in a direction substantially perpendicular to a bottom of the vessel 10. The liquid 12 stored in the vessel 10 is, for example, 3-methylaluminum.
A first float 30 is disposed around the first guide pole 20 and is supported so as to be movable up-and-down along the first guide pole 20. The first float 30 can float on a surface of a liquid 12 and thus assume a position corresponding to the level of the liquid 12. However, a range over which the first float 30 can move along the first guide pole 20 is restricted by a pair of first stoppers 50 attached to an outer surface of the first guide pole 20 at a central portion thereof. In addition, the first float 30 comprises a first magnet (not shown). A first magnetic sensor 40 is disposed inside a central portion of the first guide pole 20 midway between the stoppers 50 so as to sense an arrival of the first magnet at a middle portion of the vessel 10.
A second float 32 and a third float 34 are disposed along the second guide pole 22, and are supported so as to be movable up-and-down along the second guide pole 22. The second and third floats 32 and 34 can float on the surface of the liquid 12 and thus assume positions according to the level of the liquid 12. However, a range over which the second float 32 can move along the second guide pole 22 is restricted by a pair of second stoppers 52 attached to an outer surface of the second guide pole 22 at an upper portion thereof. In addition, the second float 32 comprises a second magnet (not shown), and a second magnetic sensor 42 is disposed inside the upper portion of the second guide pole 22 midway between second stoppers 52 so as to sense an arrival of the second magnet at an upper portion of the vessel 10. Similarly, a range over which the third float 34 can move along the second guide pole 22 is restricted by a pair of third stoppers 54 attached to an outer surface of the second guide pole 22 at a lower portion thereof. The third float 34 comprises a third magnet (not shown), and a third magnetic sensor 44 is disposed inside the second guide pole 22 midway between third stoppers 54 so as to sense an arrival of the third magnet at a lower portion of the vessel 10. Thus, the second magnetic sensor 42 detects a state in which the level of liquid 12 in the vessel 10 is high, the first magnetic sensor 40 detects a state in which the level of liquid 12 is at a middle portion of the vessel 10, and the third magnetic sensor 44 detects a state in which the level of the liquid 12 in the vessel 10 is low.
However, a spacing between the above-mentioned sensors 40, 42, and 44 is relatively wide. Thus, there are some points where the level of the liquid 12 may not be detected. Additionally, the floats 30, 32, and 34 can be moved by the bubbling of the liquid 12 when the liquid is converted to a source gas. In that case, the level of the liquid 12 may not be detected precisely. Additionally, the above-mentioned magnetic sensors 40, 42, and 44 may malfunction, in which case the magnetic sensors 40, 42, and 44 do not detect a state in which the liquid 12 is depleted. If this occurs and the vessel 10 is not replaced, the deposition process will fail. Thus, it is difficult to manage an operation of replacing the reservoir 10 when a liquid level measuring apparatus of the type shown in FIG. 1 is employed.